The technique involved genetically engineering the mice to
contain a green fluorescent protein. The protein was created to
react to the presence of calcium ions so, when the neuron fired and
the cell naturally flooded with those ions, the cells fluoresced
green.

A little microscope positioned just above the hippocampus in the
mouse's brain could then capture the activity and send it to a
computer screen for near real-time monitoring as the mouse runs
around a little arena.

"We can literally figure out where the mouse is in the arena by
looking at these lights," said biologist Mark Schnitzer, senior author on the paper which
has been published in the journal Nature Neuroscience.

"The hippocampus is very sensitive to where the animal is in its
environment, and different cells respond to different parts of the
arena. Imagine walking around your office. Some of the neurons in
your hippocampus light up when you're near your desk, and others
fire when you're near your chair. This is how your brain makes a
representative map of a space."

These patterns of firing in the mouse brain were found to stay
consistent even after weeks had passed between tests. This
consistency is what makes it possible to use the technique as a
tool with which to study progressive brain diseases and evaluate
the effectiveness of some types of treatment and therapy.